EXCHANGE 


8   1920 

University  of  Texas  Bulletin 

No.  1855:  October  1,  1918 


The  Strength  of  Fine- Aggregate  Concrete 


BY 


F.  E.  GIESECKE,  H.  R.  THOMAS,  AND  G.  A.  PARKINSON 


BUREAU  OF  ECONOMIC  GEOLOGY  AND  TECHNOLOGY 

J.  A.  Udden,  Director 

ENGINEERING  DIVISION 

F.  E.  Giesccke,  Head  of  the  Division 


PUBLISHED  BY 

THE  UNIVERSITY  OF  TEXAS 
AUSTIN 


m 


Publications  of  the  University  of  Texas 

Publications  Committee: 

F.  W.  GRAFF  R.  H.  GRIFFITH 

G.  C.  BUTTE  J.  L.  HENDERSON 
D.  B.  CASTEEL          E.  J.  MATHEWS 
FREDERIC  DUNCALF  C.  E.  ROWE 


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975-4760-620-2m 


University  of  Texas  Bulletin 

No.  1855:  October  1,  1918 


The  Strength  of  Fine-Aggregate  Concrete 


BY 


F.  E.  GIESECKE,  H.  R.  THOMAS,  AND  G.  A.  PARKINSON 


BUREAU  OF  ECONOMIC  GEOLOGY  AND  TECHNOLOGY 

J.  A.  Udden,  Director 

ENGINEERING  DIVISION 

F.  E.  Giesecke,  Head  of  the  Division 


PUBLISHED  BY  THE  UNIVERSITY  SIX  TIMES  A  MONTH,  AND  ENTERED  AS 

SECOND-CLASS  MATTER  AT  THE  POSTOFFICE  AT  AUSTIN,  TEXAS. 

UNDER  THE  ACT  OF  AUGUST  24,  1912 


The  benefits  of  education  and  of 
useful  knowledge,  generally  diffused 
through  a  community,  are  essential 
to  the  preservation  of  a  free  govern- 
ment. 

Sam  Houston 


Cultivated    mind    is    the    guardian 

genius  of  democracy It  is  the 

only  dictator  that  freemen  acknowl- 
edge and  the  only  security  that  free- 
men desire. 

Mirabeau  B.  Lamar 


EXCHANGE 


THE  STRENGTH  OF  FINE-AGGREGATE  CONCRETE 
I.      j  NTROQD'UCJf ION  \  '  : ' 


During  the  meeting- of  the-  iiexasiR<5adl-  Builders'  Asso- 
ciation in  Austin,  February  18  to  20,  1920,  the  general 
discussion  of  concrete  construction  developed  the  facts  that 
in  certain  sections  of  Texas  the  available  local  concrete 
aggregate  is  rather  fine;  that  coarse  aggregate  can  be  se- 
cured only  at  a  very  high  cost;  and  that,  under  such  condi- 
tion, it  might  be  well  to  prepare  concrete  withdut  the  use 
of  coarse  aggregate,  increasing,  when  necessary,  the  cement- 
aggregate  ratio  so  as  to  secure  the  desired  physical  proper- 
ties in  the  concrete. 

This  condition  suggested  the  need  for  an  investigation  to 
determine  how  the  compressive  strength  of  the  concrete 
varies  with  the  relative  quantity  of  cement  when  the  max- 
imum size  of  the  aggregate  used  in  the  preparation  of  the 
concrete  is  considerably  smaller  than  that  usually  employed 
in  concrete  construction.  Accordingly,  four  series  of  tests 
were  made  in  the  manner  and  with  the  results  described 
in  this  bulletin. 

II.      SCOPE    OF    THE    INVESTIGATION 

The  object  of  the  investigation  was  to  determine  the  ulti- 
mate compressive  strength  at  28  days  of  concrete  prepared 
of  fairly  well  graded  aggregate  the  maximum  size  of  which 
varied  from  one-eighth  to  one-half  inch.  In  addition,  tests 
were  made  for  the  purpose  of  determining  something  of  the 
effect  of  additional  water  on  the  strength  of  the  concrete, 
and  also  the  effect  of  rodding  the  concrete  in  which  excess 
water  was  used.  A  few  tests  were  made  for  the  purpose 
of  comparing  the  strength  of  6  by  12-inch  cylinders  with 
that  of  the  2  by  4-inch  cylinders  used  for  the  majority  of 
the  specimens. 


977632 


4  University  of  Texas  Bulletin 

III.      MATERIALS    AND    METHOD    OF    TESTING 

Aggregate. — Tlie.  aggregate  ^used  consisted  of  Colorado 
River  sand  and*gi:ivelv  "the  ^'Janulometrie  composition  of 
which  is 'shown:in.tbe  accompanying  table. 

SIEVE  ANALYSES  OF  AGGREGATES 
Material  Passing  the  Various  Sieves,  in  Percentage  by  Weight 

Sieve  Maximum  Size  of  Aggregate — Inches 

Size  Vs  14  %  Vz 


*& 







100.0 

% 





100.0 

82.7 

y* 

____ 

100.0 

75.4 

63.5 

Vs 

100.0 

76.9 

53.0 

46.4 

14 

49.6 

39.0 

35.2 

32.5 

28 

32.8 

29.6 

27.1 

25.3 

48 

4.4 

4.1 

4.0 

4.0 

100 

0.9 

0.6 

0.8 

0.7 

200  0.9  0.0  0.0  0.0 

Cement. — The  cement  was  a  blend  of  three  brands  of 
Texas  cement.  The  water  required  for  normal  consistency 
was  23  per  cent.  Fineness :  18  per  cent  retained  on  No.  200 
sieve.  Time  of  set :  initial  set  in  2  hours,  40  minutes ;  final 
set  in  7  hours.  Average  tensile  strength  of  1 :  3  mortar 
briquettes  made  with  standard  Ottawa  sand  was  293  pounds 
per  square  inch  at  7  days  and  378  pounds  per  square  inch 
at  28  days. 

Water  for  Sand. — The  quantity  of  water  used  in  prepar- 
ing the  specimens  was  based  on  the  granulometric  compo- 
sition of  the  aggregate  according  to  a  system  developed  in 
this  laboratory  by  Mr.  G.  A.  Parkinson,  Assistant  Testing 
Engineer,  this  system  being  used  in  our  regular  work  unless 
a  different  method  is  specified.  By  a  series  of  experiments 
Mr.  Parkinson  determined,  for  example,  that  for  a  sand 
passing  a  No.  40  sieve  and  retained  on  a  No.  50  sieve  an 
apparent  normal  consistency  was  obtained  when  the  mixing 
water  was  8.5  per  cent  of  the  weight  of  the  sand,  in  addition 
to  the  water  which  was  used  for  the  cement  and  which 


The  Strength  of  Fine- Aggregate  Concrete  5 

amounted  to  23  per  cent  (normal  consistency)  of  the  weight 
of  the  cement.  Other  proportions  were  determined  in  a 
similar  way  for  sands  of-  other  degrees  of  fineness.  The 
final  result  is  shown  in  Figure  1,  in  which  the  percentage 
(by  weight)  of  mixing  water  is  plotted  against  the  mean 
diameter  of  the  grains  of  the  aggregate,  the  mean  diameter 
being  taken  as  the  mean  diameter  of  the  sieves  passed  and 
retained  on.  Knowing,  then,  the  sieve  analysis  of  the  ag- 
gregate, the  quantity  of  mixing  water  may  be  determined 
from  the  curves  given.  In  this  bulletin,  the  specimens  made 
up  with  water  determined  in  this  way  will  be  referred  to  as 
having  normal  water  content. 


aos  a/o 

Mean  Diameter  of  5ar?d  fizrf/c/es  -//? 


a/s 


FIG.  1.  —  Water-for-sand  curve,  showing  amount  of  water  added  for 
sand,  based  on  sieve  analysis 

For  example,  the  water  for  the  Vs-inch  maximum  aggre- 
gate was  computed  as  shown  in  the  following  table  for  a 
total  weight  of  100  grams  (or  pounds)  of  aggregate. 


University  of  Texas  Bulletin 

9 

Water,  in  Weight 

Sieve  Number        Weight  of  Sand  Percentage  of  Water 

Passed    Retained    Between  Sieves  by  Weight  Required 

%                 10                 37.7  5.0  1.89 

10                 20                 19.4  5.5  1.06 

20                 30                 20.5  6.5  1.33 

30                 40                 12.2  7.5  0.92 

40                 50                   6.1  8.5  0.52 

50                 60                   1.6  9.5  0.15 

60                 80                   1.0  11.5  0.12 

80               100                   0.5  13.5  0.07 

100               200                   0.5  20.0  0.10 

200                                       0.5  25.0  0.13 


Total  water  for  sand 6.29  g.  (Ib.) 

If  1200  grams  of  sand  are  to  be  used  (as  was  the  case  in 
these  mixes)  the  total  water  will  be  12x6.29,  equals  75.5  g. 
In  addition  to  this  amount  of  water  there  must  be  added  the 
water  for  the  cement,  which  amounted  to  23  per  cent  of  the 
weight  of  the  cement. 

It  is  very  probable  that  the  percentage  of  mixing  water 
required  depends  not  only  on  the  granulometric  composition 
of  the  aggregate  but  also  on  the  mineralogical  constitution 
of  the  aggregate  and  that,  therefore,  the  diagram  of  Fig- 
ure 1  can  not  be  used  in  all  cases ;  it  is  used  in  this  labora- 
tory for  silica  sands.  For  more  absorbent  sands  sufficient 
additional  water  should  be  added  to  allow  for  the  quantity 
of  water  absorbed  by  the  sand  in  two  or  three  hours. 

Test  Specimens. — The  test  specimens  were  cylindrical 
and  approximately  2  by  4  inches  in  size,  except  that  a  few 
specimens  were  made  in  the  form  of  6  by  12-inch  cylinders. 
For  the  small  cylinders,  the  concrete  for  each  batch,  com- 
prising three  specimens,  was  mixed  by  hand  and  placed  into 
the  molds  in  layers  of  one  inch,  each  layer  being  tamped 
with  a  steel  tamp  one  inch  in  diameter  and  weighing  about 
three-fourths  pound.  This  method  is  the  one  recommended 
by  the  American  Society  for  Testing  Materials  in  their  ten7 
tative  standard  for  compressive  tests  of  mortars.  In  mak- 
ing the  6  by  12-inch  cylinders,  the  material  for  a  single  spec- 
imen was  mixed  by  hand  with  a  trowel  in  a  shallow  pan. 


The  Strength  of  Fine-Aggregate  Concrete  7 

The  molding  of  these  cylinders  was  similar  to  the  method 
used  for  the  smaller  ones,  except  that  a  larger  tamping  bar 
was  used,  and  that  the  material  was  tamped  in  3-inch  lay- 
ers. At  the  time  of  making  these  specimens  it  was  felt  that 
it  was  impossible  to  secure  as  thorough  mixing  with  a 
trowel  as  was  possible  when  the  mixing  was  done  with  the 
hands,  and  the  testing  of  these  specimens  showed  this  to  be 
the  case,  as  will  be  seen  later. 

The  2  by  4-inch  cylinders  were  stored  in  a  damp-closet 
for  24  hours  at  the  end  of  which  time  they  were  removed 
from  the  molds  and  stored  in  water  for  27  days.  The  larger 
cylinders  were  capped  with  a  glass  plate  to  prevent  drying 
out  and  were  left  in  the  molds  for  24  hours  at  the  end  of 
which  time  they  were  removed  from  the  molds  and  stored 
in  water  for  27  days.  Before  testing,  all  cylinders  were 
capped  with  plaster  of  Paris. 

Four  series  of  specimens  were  prepared,  differing  from 
each  other  in  the  maximum  size  of  the  aggregate,  the  sizes 
being  Vs>  1/4,  %,  and  1/2  inch.  In  each  series,  varying 
cement-aggregate  ratios  were  used  so  that  the  resulting 
mixes  varied  from  fairly  lean  to  very  rich.  For  each  mix, 
three  specimens  were  made. 

In  order  to  be  able  to  compare  the  results  obtained  with 
the  2  by  4-inch  specimens  with  those  obtained  with  6  by  12- 
inch  specimens,  one  6  by  12-inch  specimen  was  prepared 
for  each  series,  using  the  mix  in  each  case  that  corresponded 
approximately  to  ten  sacks  of  cement  in  a  cubic  yard  of 
concrete. 

Since  it  is  not  practicable  to  prepare  concrete  in  the  field 
with  as  little  mixing  water  as  was  used  in  this  investigation, 
one  of  the  mixes  of  each  series  was  duplicated,  using,  how- 
ever, 30  per  cent  more  water  than  in  the  normal  mix.  The 
mix  chosen  for  these  tests  was  that  corresponding  to  prac- 
tically ten  sacks  of  cement  in  a  cubic  yard  of  concrete.  The 
resulting  concrete  was  at  wet  as  could  conveniently  be 
worked  on  the  glass  mixing  plate. 

It  has  been  found  in  this  laboratory  that  concrete,  par- 
ticularly when  it  contains  excess  water,  can  be  materially 


8  University  of  Texas  Bulletin 

improved  by  rodding  after  it  has  been  deposited  in  the 
forms.  To  obtain  additional  information  on  the  increase 
in  strength  due  to  rodding,  the  specimens  mixed  with  30 
per  cent  excess  water  were  made  in  duplicate,  half  of  them 
being  tamped  and  half  rodded.  The  rodding  consisted  of 
forcing  a  i/i-inch  pointed  steel  rod  into  the  concrete  as  it 
was  being  placed  in  the  molds,  and  afterwards  at  intervals 
of  30  minutes.  Each  rodding  was  continued  for  15  sec- 
onds, during  which  time  about  20  strokes  of  the  rod  were 
made.  The  specimens  containing  30  per  cent  excess  water 
were  rodded  six  times.  It  should  be  noted  that  as  the  water 
rose  to  the  surface  due  to  rodding,  it  was  poured  off  instead 
of  allowing  it  to  be  re-absorbed  by  the  concrete. 

Supplementary  Tests. — After  having  tested  the  speci- 
mens mixed  with  excess  water  it  was  found  that  the  results 
for  the  series  in  which  the  Vs-inch  maximum  aggregate 
was  used  differed  so  much  from  those  of  the  other  series 
that  it  was  decided  to  disregard  the  results  for  that  series 
and  to  repeat  the  entire  set  of  tests  in  which  excess  water 
was  used. 

In  repeating  this  series,  60  specimens  were  prepared; 
twelve  of  this  number  were  mixed  with  the  normal  quantity 
of  water  for  reference ;  24  were  mixed  with  30  per  cent  ex- 
cess water  and  24  with  40  per  cent  excess  water.  Of  the 
48  specimens  mixed  with  excess  water  12  of  each  group 
were  tamped  and  12  of  each  group  were  rodded  exactly  as 
described  for  the  original  series  with  excess  water,  except 
that  the  number  of  roddings  was  increased  from  six  to 
eight  in  every  case. 

The  60  specimens  of  this  series  were  prepared  as  de- 
scribed for  the  original  series  except  that  each  specimen 
for  the  normal  water  content  was  mixed  separately,  while 
those  mixed  with  excess  water  were  made  in  pairs,  one  of 
which  was  tamped  and  the  other  rodded.  The  three  speci- 
mens of  each  group  were  prepared  on  different  ^days. 


The  Strength  of  Fine-Aggregate  Concrete  9 


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FIG.  2. — Showing  the  relation  of  the  compressive  strength  of  concrete 
to  the  relative  quantity  of  cement 


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University  of  Texas  Bulletin 


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FIG.  3.  —  Showing  the  relation  of  the  compressive  strength  of  concrete 
to  the  relative  quantity  of  cement 


The  Strength  of  Fine- Aggregate  Concrete  11 

IV.      RESULTS   OF   TESTS 

Specimens  with  Normal  Water. — The  results  of  the  four 
series  of  tests  are  shown  graphically  in  Figures  2  and  3, 
in  which  the  ultimate  unit  compressive  strength  at  28  days 
is  plotted  against  the  number  of  sacks  of  cement  contained 
in  a  cubic  yard  of  concrete. 

For  ease  of  comparison,  the  results  obtained  for  the  four 
sizes  of  aggregate  and  normal  amount  of  water,  shown  in 
Figures.  2  and  3,  have  been  replotted  in  the  upper  diagram 
of  Figure  4.  It  is  evident  from  this  diagram  that  to  obtain 
an  ultimate  strength  at  28  days  of  3000  pounds  per  square 
inch,  for  example,  7.5  sacks  of  cement  must  be  used,  per 
cubic  yard  of  concrete  if  the  aggregate  is  graded  up  to 
one-eighth  inch,  and  6.1,  4.8,  and  4.6  sacks  per  yard  if  the 
aggregate  is  graded  up  to  one-fourth,  three-eighths,  and 
one-half  inch,  respectively. 

Comparing  the  results  obtained  for  the  8-sack  mix,  for 
example,  we  find  strengths  of  3300,  4400,  5700,  and  6000 
pounds  per  square  inch  when  the  aggregate  is  graded  up  to 
one-eighth,  one-fourth,  three-eighths,  and  one-half  inch, 
respectively. 

The  lower  diagram  of  Figure  4  was  derived  from  the 
upper  diagram  by  reading  from  the  curves,  for  a  given 
cement  content,  the  strengths  obtained  for  the  several  sizes 
of  aggregates.  These  values  were  then  plotted  against  the 
corresponding  sizes  of  the  aggregate.  From  this  diagram 
it  is  evident  that,  up  to  a  certain  point,  for  a  given  quantity 
of  cement  a  stronger  concrete  will  be  obtained  for  the 
coarser  material  than  for  the  finer  aggregate.  In  order  to 
obtain  concrete  of  a  given  strength  it  is  evidently  a  question 
of  whether  it  would  be  cheaper  to  use  the  finer  local  material 
with  additional  cement,  or  to  pay  a  higher  price  (if  neces- 
sary) to  obtain  a  coarser  aggregate  which  will  give  the  de- 
sired strength  with  a  smaller  proportion  of  cement. 

In  using  these  values  it  must  be  remembered  that  the 
strength  of  field  concrete  is  generally  less  than  that  of  lab- 
oratory concrete.  But  from  comparative  tests  made  a  few 


12 


University  of  Texas  Bulletin 


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FlG.  4.  —  Curves  showing  the  variation  in  the  strength  of  concrete  with 

the  variation  in  the  maximum  size  of  aggregate  and  the 

relative  quantity  of  cement 


The  Strength  of  Fine-Aggregate  Concrete  13 

years  ago  by  this  laboratory  it  is  believed  that  builders 
should  be  able  to  secure  a  strength  in  their  field  concrete 
which  is  at  least  two-thirds  of  that  which  would  be  obtained 
with  the  same  materials  under  laboratory  conditions.  That 
is,  if  it  is  desired  to  determine  the  amount  of  cement  which 
would  be  necessary  to  produce  a  2000-pound  concrete  under 
field  conditions,  it  would  be  necessary  to  take  from  this  dia- 
gram the  amount  of  cement  necessary  to  produce  a  strength 
of  3000  pounds  per  square  inch  under  laboratory  conditions. 
It  should  not  be  forgotten,  however,  that  the  diagram  may 
not  apply  if  the  grading  of  the  material  is  different  from 
that  of  the  aggregate  used  for  these  tests  or  if  the  character 
of  the  aggregate  is  different  from  that  here  reported. 

In  all  important  work  it  is  advisable  to  prepare  compara- 
tive specimens  of  the  actual  aggregate  or  aggregates  to  be 
used,  with  varying  proportions  of  cement,  in  order  to  deter- 
mine the  most  economical  proportions. 

The  amount  of  water  used  should  be  accurately  controlled 
and  should  be  the  minimum  which  will  produce  a  workable 
mix. 

Large  Specimens. — In  Figures  2  and  3  are  also  shown  the 
strengths  of  6  by  12-inch  specimens.  It  should  be  noted 
that  the  strengths  of  these  specimens  were  less  than  for  the 
corresponding  2  by  4-inch  specimens,  ranging  from  6  per 
cent  less  for  the  Vs-incn  maximum  aggregate  to  32  per  cent 
less  for  the  i/2-inch  maximum  aggregate.  As  has  previously 
been  pointed  out,  this  was  evidently  due  to  the  inferior  mix- 
ing produced  by  trowel  to  that  produced  by  hand. 

Specimens  with  Excess  Water;  First  Series. — It  is  also  ap- 
parent from  Figures  2  and  3  that  the  average  strength  of 
the  tamped  specimens  in  which  30  per  cent  excess  water 
was  used  was  about  21  per  cent  less  than  the  normal  mix 
for  the  14-inch  maximum  aggregate,  38  per  cent  less  for  the 
%-inch  maximum  aggregate,  and  40  per  cent  less  for  the 
i/^-inch  maximum  aggregate. 

Comparing  the  rodded  specimens  containing  30  per  cent 
excess  water  with  the  normal  tamped  specimens,  we  find 
that  the  rodded  specimens  are  6.0  per  cent  stronger  than  the 


14  University  of  Texas  Bulletin 

normal  for  the  14 -inch  maximum  aggregate,  8.8  per  cent 
weaker  for  the  %-inch  maximum  aggregate,  and  14.0  per 
cent  weaker  for  the  i/2-inch  maximum  aggregate. 

Comparing  the  rodded  specimens  containing  30  per  cent 
excess  water  with  the  tamped  ones  containing  30  per  cent 
excess  water,  we  find  that  the  rodded  specimens  are  35  per 
cent  stronger  than  the  tamped  for  the  14-inch  manimum 
aggregate,  46  per  cent  stronger  for  the  %-inch  maximum 
aggregate,  and  43  per  cent  stronger  for  the  %-inch  maxi- 
mum aggregate. 

Specimens  with  Excess  Water;  Supplementary  Series. — 
In  the  upper  diagram  of  Figure  5  are  given  results  of 
the  supplementary  tests.  Each  point  indicated  is  the  aver- 
age for  three  tests.  It  will  be  noted  that  three  curves  are 
given :  one  for  the  specimens  made  with  the  normal  amount 
of  water,  one  for  specimens  made  with  30  per  cent  excess 
water  and  tamped  into  the  molds,  and  one  for  specimens 
made  with  30  per  cent  excess  water  and  rodded  as  already 
described. 

In  the  lower  diagram  of  Figure  5  are  given  curves  for 
tests  of  the  specimens  made  with  40  per  cent  excess  water, 
both  rodded  and  tamped.  For  ease  of  comparison,  the  curve 
for  normal  water  content  has  been  repeated  from  the  upper 
diagram. 

Comparing  the  rodded  specimens  made  with  30  per  cent 
excess  water  with  the  tamped  specimens  made  with  normal 
water  content,  we  find  that  the  rodded  specimens  are  15  per 
cent  stronger  than  the  normal  for  the  i/s-inch  maximum 
aggregate,  5  per  cent  stronger  for  the  1/4 -inch  maximum 
aggregate,  and  3  per  cent  weaker  for  both  the  %  and  %-inch 
maximum  aggregate. 

Comparing  the  rodded  specimens  made  with  30  per*  cent 
excess  water 'with  the  tamped  specimens  with  30  per  cent 
excess  water,  we  find  the  rodded  specimens  32  per  cent 
stronger  than  the  tamped  for  Vs-inch  maximum  aggregate, 
and  34,  37,  and  30  per  cent  stronger  for  %,  %,  and  y%-mch 
maximum  aggregate,  respectively. 


The  Strength  of  Fine- Aggregate  Concrete  15 


800O 


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30%  £xce&  Water  -ffati/ed 


J/ze  of  rfggregafe  -//? 


8000 


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7000 
^6000 


woo 


JOOO 


2000 


f/orma/- 


s 


of  Sftree 


Norma* r  M/x 

4OZ£x.ce&Watfe 

40%  Excett  Wafer  -fltototeef 


te 


3/< 


<5/'ze  of 


FIG.  5.  —  Curves  showing  the  effect  of  excess  water,  with  and  without 

rodding 


16  University  of  Texas  Bulletin 

In  the  lower  diagram  of  Figure  5,  comparing  the  rodded 
specimens  made  with  40  per  cent  excess  water  with  the 
tamped  specimens  made  with  normal  water  content  we  find 
that  the  rodded  specimens  are  6  per  cent  stronger  than  the 
normal  for  the  Vs-inch  maximum  aggregate,  2  per  cent 
stronger  for  the  14 -inch  maximum  aggregate,  6  and  2  per 
cent  weaker  for  the  %  and  %-inch  maximum  aggregate,  re- 
spectively. 

Comparing  the  rodded  specimens  made  with  40  per  cent 
excess  water  with  the  tamped  specimens  with  40  per  cent 
excess  water,  we  find  the  rodded  specimens  are  43  per  cent 
stronger  than  the  tamped  for  i/g-inch  maximum  aggregate 
and  53,  42,  and  61  per  cent  stronger  for  1/4,  %,  and  i/^-inch 
maximum  aggregate,  respectively. 

The  supplementary  tests  bring  out  an  important  relation 
between  the  strength  of  concrete  and  the  amount  of  mixing 
water  used.  Comparing  the  tamped  specimens  containing 
excess  and  normal  water,  we  find  that  the  30  per  cent  ex- 
cess water  causes  an  average  reduction  in  strength  for  the 
four  types  of  aggregate  of  22  per  cent.  To  produce  this 
same  reduction  in  strength  in  the  normal  mix  by  reducing 
the  amount  of  cement  would  require  the  cement  to  be  re- 
duced from  10  sacks  per  yard  to  7.8  sacks  per  yard — a  re- 
duction in  cement  content  of  22  per  cent.  In  the  same  way 
we  find  that  the  40  per  cent  excess  water  causes  an  average 
reduction  in  strength  of  33  per  cent.  To  produce  the  same 
reduction  in  strength  of  the  normal  mix  by  reducing  the 
cement  content  would  require  the  cement  to  be  reduced  from 
10  sacks  to  6.9  sacks  per  yard — a  reduction  of  31  per  cent. 
In  other  words,  for  a  10-sack  mix  each  one  per  cent  increase 
in  the  amount  of  mixing  water  beyond  that  required  for 
normal  mix  reduces  the  strength  of  the  concrete  practically 
three-fourths  of  one  per  cent.  That  is,  it  is  quite  as  neces- 
sary not  to  add  too  much  water  as  it  is  to  use  enough 
cement. 


The  Strength  of  Fine- Aggregate  Concrete  17 

V.      SUMMARY  OF  RESULTS 

1.  Good  concrete  can  be  prepared  without  the  use  of 
coarse  aggregate. 

2.  To  obtain  concrete  of  a  given  strength  with  given  ma- 
terials, the  relative  quantity  of  cement  must  be  increased 
as  the  maximum  size  of  the  aggregate  is  decreased. 

3.  The  maximum  size  of  the  aggregate,  when  not  deter- 
mined by  other  factors,  should  be  determined  so  as  to  obtain 
the  lowest  possible  cost  of  the  concrete,  taking  into  account 
the  cost  of  the  aggregate  and  that  of  the  quantity  of  cement 
necessary  for  the  type  of  aggregate  used. 

4.  On  important  work  it  is  desirable  to  make  compara- 
tive tests,   with   various  cement-aggregate   ratios,   of  the 
available  aggregates  in  order  to  determine  which  aggregate 
should  be  used  and  the  most  economical  mix. 

5.  In  order  to  obtain  a  concrete  of  maximum  strength, 
the  amount  of  water  used  must  be  the  minimum  which  will 
produce  a  consistency  such  that  the  concrete  can  be  placed 
properly. 

6.  •  The  strength  of  concrete  can  be  increased  materially 
by  rodding  it  after  it  has  been  deposited  in  the  forms,  par- 
ticularly if  the  water  which  is  rodded  out  of  the  concrete 
can  run  off  freely  as  it  comes  to  the  surface. 


SOME  UNIVERSITY  OF  TEXAS  BULLETINS 
OF  INTEREST  TO  ENGINEERS 


Bulletin  No.  26,  May  5,  1915:    Paxton,  E.  T.,  Street  Paving  in  Texas. 

Bulletin  No.  57,  October  10,  1915:  Baker,  C.  L.,  Geology  and  Under- 
ground Waters  of  the  Northern  Llano  Estacado. 

Bulletin  No.  62,  November  5,  1915:  Nash,  J.  P.,  Road  Materials  of 
Texas. 

Bulletin  No.  1725,  May  1,  1917:     Nash,  J.  P.,  Texas  Granites. 

Bulletin  No.  1733,  June  10,  1917:  Tyler,  R.  G.,  Ed.,  Water  Supply 
and  Sanitation. 

Bulletin  No.  1735,  June  20,  1917:  Tyler,  R.  G.,  Ed.,  Roads  and 
Pavements. 

Bulletin  No.  1752,  September  15,  1917:  Read,  W.  T.,  Boiler  Waters; 
Their  Chemical  Composition,  Use  and  Treatment. 

Bulletin  No.  1759,  October  20,  1917:  Giesecke,  F.  E.,  The  Friction 
of  Water  in  Pipes  and  Fittings. 

Bulletin  No.  1771,  December  20,  1917 :  Nash,  J.  P.,  Tests  of  Concrete 
Aggregates  Used  in  Texas. 

Bulletin  No.  1814,  March  5,  1918:  Schoch,  E.  P.,  Chemical  Analyses 
of  Texas  Rocks  and  Minerals. 

Bulletin  No.  1815,  March  10,  1918:  Giesecke,  F.  E.,  and  Finch,  S.  P.: 
Physical  Properties  of  Dense  Concrete  as  Determined  by  the 
Relative  Quantity  of  Cement. 

Bulletin  No.  1855,  October  1,  1918:  Giesecke,  F.  E.,  Thomas,  H.  R., 
and  Parkinson,  G.  A.,  The  Strength  of  Fine- Aggregate  Con- 
crete. 


Copies  of  the  above  Bulletins  may  be  obtained  free  of  charge  upon 
application  to  the  Chairman  of  the  Publications  Committee,  Univer- 
sity of  Texas,  Austin. 


977632 


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